Investigation on Coded Modulation Combining 6-QAM and Two-Level LDPC Codes

Considering the problem of matching the non-standard modulation with the order of 3×2^p and error correction code, a two-level coded modulation transmission scheme for 6-order quadrature amplitude modulation (QAM) is proposed. At the transmitter, the Galois field GF(2) low-density parity-check (LDPC) code and GF(3) LDPC code were used for layered encoding, and the two kinds of LDPC codewords were mapped to senary codewords for 6-QAM modulation. At the receiver, two-level iterative decoding methods were designed according to the decoding order of GF(3) LDPC code and GF(2) LDPC code. Simulation results show that, under the additive white Gaussian noise (AWGN) channel, performing an overall iterative decoding, the coded modulation scheme for 6-QAM which performs GF(3) LDPC decoding first achieves better decoding performance than the scheme which performs GF(2) LDPC decoding first. As the number of iterations increases, the error correction performance can be further improved. Comparing 6-QAM with 6-order phase shift keying (6-PSK), if performing GF(3) LDPC decoding first, the transmission scheme combining 6-QAM and two-level LDPC codes can obtain about 1.3 dB gain under the AWGN channel at SER of 10^-5 .